Related Art Statement
This invention relates to a sound reproducing apparatus in which the reproduced sound from a loudspeaker unit is heard via an acoustic tube attached to an auricle. More particularly, it relates to a sound reproducing apparatus in which the reproduced sound from the loudspeaker unit is radiated into an acoustic tube having one end arranged as an auricular attachment section and the other end arranged as a non sound-reflecting end, and in which the reproduced sound is heard by way of the acoustic tube.
Various types of sound reproducing apparatus, are known such as the earphone or headphone type apparatus, in which the reproduced sound is heard with a loudspeaker unit attached to the listener's auricle.
When the reproduced sound is heard with the sound reproducing apparatus now in extensive use, such as an earphone, the reproduced sound radiated from the loudspeaker unit may be heard as the reproduced sound radiated from the loudspeaker unit is radiated from an earphone casing accommodating the loudspeaker unit through the external acoustic meatus to set the tympanic membrane into oscillations.
In the above earphone, the reproduced sound reaching the tympanic membrane tends to be reflected thereat and to exit the auricle by way of the external acoustic meatus. However, in the conventional earphones, the earphone casing is attached to the auricle as it encloses the external acoustic meatus, so that the reflected sound tending to exit the auricle is reflected by the casing or the loudspeaker unit therein so as to be radiated again into the external acoustic meatus to travel towards the tympanic membrane.
Hence, in the above described conventional earphones, the sound directly radiated from the loudspeaker unit towards the tympanic membrane and the sound once reflected by the tympanic membrane and again reflected at the earphone casing or at the loudspeaker unit are heard by the listener.
When the reproduced sound directly radiated towards the tympanic membrane and the sound reflected by the tympanic membrane are temporally spaced apart by a period not more than several hundred microseconds, the sounds are heard as if the sound source is within the user's head so as to impart a "fixed" or "oppressed" feeling to the listener.
In order to prevent the sound once reflected by the tympanic membrane from being reflected again by the earphone housing, there is proposed an earphone in which a central portion of a diaphragm of a loudspeaker unit provided facing towards the external acoustic meatus of the auricle is removed so that the reproduced sound is radiated only from the periphery of the diaphragm. In such case, the sound directly reflected onto the earphone housing may be reduced, while the region of the diaphragm set into oscillations by the reflected sound may be lessened, resulting in a reduced amount of the reflected sound re-radiated towards the diaphragm, and improved sound reproducing characteristics.
However, when the sound source at the loudspeaker unit side is seen from the external acoustic meatus, the acoustic impedance at the entrance to the external acoustic meatus differs from that within the external acoustic meatus resulting again in sound reflection at an area between the earphone casing and the entrance to the external acoustic meatus, with the reflected sound being re-directed towards the tympanic membrane Thus the above mentioned "fixed" or "oppressed" feeling cannot be avoided
There is also proposed an earphone in which the sound reflection by the earphone casing or the like is avoided by reverse filter techniques.
However, the reverse filter techniques present disadvantages in that they are not universally applicable since the acoustic impedance of the tympanic membrane is not the same for individual users and hence the reverse filter needs to be tailored to each user.
Thus there is devised a sound reproducing apparatus in which, in order to prevent sound reflection at the loudspeaker unit or the earphone casing on the sound source side as the sound source side is viewed from the entrance to the external acoustic meatus, or sound reflection caused by impedance changes at the entrance to the external acoustic meatus, the reproduced sound radiated from the loudspeaker unit is transmitted to the auricle by way of an acoustic tube.
Meanwhile, in this type of the sound reproducing apparatus employing an acoustic tube, a loudspeaker unit is provided on one lateral side of the acoustic tube. As such a loudspeaker unit, a loudspeaker having a diaphragm is generally employed.
However, in general, the acoustic impedance of a diaphragm employed in a diaphragm type loudspeaker is lower than that within the inside of the acoustic tube. On the other hand, it is difficult to attach the diaphragm type loudspeaker to the tube, with the sound radiating surface thereof directed to the inside of the tube, without changing the inside diameter of the acoustic tube. Therefore, when the loudspeaker unit is simply mounted on one side of the acoustic tube, sound reflection occurs at the site of the diaphragm of the loudspeaker unit.